A CoMP (coordinated multipoint transmission/reception) technology refers to a new technology in which multiple geographically-separated base stations coordinately transmit data to be sent to one terminal or jointly receive data sent by one terminal. The technology can effectively improve service quality for cell edge users, and therefore the technology is widely used in various transmission systems.
In a TD-LTE (Time Division-Long Term Evolution) system that uses the CoMP technology, an uplink channel and a downlink channel in the TD-LTE system use a same frequency band, and therefore have channel reciprocity. However, in a practical system, reciprocity between the uplink channel and the downlink channel is not absolute, that is, a reciprocity error exists, which may damage performance of a TDD system. In application of the CoMP technology, multiple base stations need to send data to same user equipment. Therefore, in addition to reciprocity calibration that needs to be performed between multiple antennas inside a base station, reciprocity calibration between the base stations with respect to the user equipment needs to be considered.
There is a method for reciprocity calibration between base stations in the prior art. One user equipment is selected, and the user equipment may send a pilot signal to each base station by using an air interface channel that exists between the user equipment and the base station, so that the base station estimates, according to the pilot signal, an uplink channel estimation matrix corresponding to the base station, and then the user equipment estimates, according to a pilot signal sent by each base station, a downlink channel estimation matrix corresponding to the base station, and feeds back, to the base station, the downlink channel estimation matrix corresponding to the base station. In this way, each base station may obtain, by using uplink/downlink channel estimation matrix information corresponding to the base station, a calibration matrix corresponding to the base station, and then, reciprocity calibration between base stations with respect to the user equipment is performed.
In a process of implementing the foregoing reciprocity calibration between base stations, the inventor finds that during reciprocity error calibration between the base stations, because of a relatively far distance between the base stations, and relatively poor quality of an air interface channel from the user equipment to each base station, precision of an obtained channel estimation matrix is extremely low, which causes extremely low precision of reciprocity calibration between the base stations, and causes an extremely heavy system performance loss.